The intrasanguineous host-mediated assay in mice and rats and perfused rat liver and lung were examined as systems for metabolizing mutagens for Saccharomyces cerevisia D4. There was no observable histological damage in the liver, lungs, kidneys, and testes up to 3 days after I.V. injection of yeast into mice. The yeast cells were trapped in the capillaries of the various organs, with a few phagocytized cells. the metabolism of dimethylnitrosamine (DMN) to a mutagen was compared in the host-mediated assay, perfused liver, and liver homogenate systems. Mice were more effective than rats in converting DMN to a mutagen and the host mediated and perfusion techniques were more effective than liver homogenates in vitro.